In conventional centrifuges, rotation torque obtained from a power generator such as an electric motor or the like is transmitted to a rotor via a rotational drive shaft to rotate the rotor. A plurality of test tubes each encapsulating a sample can be set on the rotor, and the samples in the test tubes are subjected to centrifugal separation by rotation of the rotor. Angle rotors, swing rotors, and the like are used for the centrifuges of this kind. The amounts of samples vary according to test tubes, depending on the extent of collection of blood or the like. Therefore, when a plurality of test tubes thus enclosing samples of different masses are set on the rotor, the barycenter of the whole rotor and test tubes tends to deviate from the rotational axis, i.e., there must be so-called mass eccentricity.
As shown in FIG. 10, an angle rotor A has a round circular shape, as a whole, and a plurality of test-tube insertion holes are formed at a predetermined angle to the rotation axis X. The angle rotor A has a relatively small size and does not undergo severe restriction of centrifugal stress due to the dead weight of itself. In addition, high processing accuracy can be attained because the angle rotor is manufactured by machining. Also, the tendency toward mass eccentricity is low during rotation because the number of attachable test tubes is small. As a result, the angle rotor A used for a centrifuge called a multi-cooling centrifuge which has the maximum rotation speed of 10,000 rpm or so cannot centrifuge much amounts at once but is suitable for centrifugation within a high-speed rotation range of, for example, 6,000 rpm to 10,000 rpm. Although the definition of the high-speed rotation rotor is not clear, the high-speed rotation rotor is used for centrifugation or separation of samples which require high centrifugal acceleration.
On the other side, as shown in FIG. 11, a plurality of (four) arms extend horizontally in radial directions from the axis X in case of a swing rotor S. The distal end of each arm is branched into two. Pins are provided in directions of opposite branches of adjacent arms. A bucket having a shape like a bottomed cylinder is pivotally movably supported to each pair of pins. Each bucket is provided with engaging portions corresponding to the pins. The engaging portions are detachable from the pins. A rack in which a plurality of test-tube holes are formed is provided in each bucket. Test tubes are inserted in these test-tube holes. In operation of centrifugation, buckets are engaged with and hang on all of the arms by the pins while two buckets are omitted from FIG. 11 to help easy understanding. When the swing rotor S set on the centrifuge reaches a predetermined rotation speed, each bucket pivotally moves to the horizontal direction about the pins due to centrifugal force, and thus, componential separation is performed on the samples.
The swing rotor S is larger as compared with the angle rotor A, and is advantageous in that a greater number of test tubes can be inserted. However, the greater number of inserted test tubes means also a tendency to greater variants among the samples filled in the test tubes. The swing rotor has a large radius of rotation and a center portion has a relatively small mass. Further, each arm of the swing rotor S has a complicated shape, and is manufactured by casting for the purpose of reduction in manufacturing-cost in many cases. In these cases, the dimensional accuracy is lower than that of the angle rotor A. Further, each bucket is detachable from the pins, so that there is a rattling between the engaging portions of the buckets and the pins. As mentioned above, centrifugal stress generated at the pins and the engaging portions is large in the swing rotor S. Further, there are great variants among samples, considering the mass. Consequently, tendency toward mass eccentricity is greatly higher as compared with the angle rotor A, and is thus not suitable for high-speed rotation. Therefore, the swing rotor is used mainly for centrifugal separation dealing with a large number of test samples within a low-speed rotation range of, for example, 2,000 to 5,000 rpm.
Next, a rotation drive shaft of a centrifuge will be described in relation to reaction force from a bearing. In case of making a large rotor usable at a low rotation speed, rigidity of the rotation shaft is increased and the flexural vibration frequency is shifted to a higher speed than a normal operating range, to improve the operation ability with high rigidity. A specific description will now be made on the basis of FIG. 12. FIG. 12 shows a rotating situation in which a rotor R1 is coaxially connected to a high-rigidity shaft S1, and the high-rigidity shaft S1 is also coaxially connected to an output shaft of a motor M as a power generator. The high-rigidity shaft means a shaft which is rigid within a range of operational rotation speed. At this time, the output shaft of the motor M is supported on a motor housing or the like not shown, by a bearing B. Where the mass of the rotor R1 is m, deviation of an actual barycenter position from a geometric center is ε which is induced from imbalance of the rotor, the rotational angular speed is ω, flexure of the high-rigidity shaft S1 is ρ, and flexural rigidity of the high-rigidity shaft S1 is k, centrifugal force Fs induced by the imbalance is expressed as follows, according to “Dynamics of rotary member” co-authored by R. Gasch and H. Pfutzner, translated by Shuzo Miwa, published by Morikita-shuppan”.Fs=mεω2The bearing reaction force Fu is obtained as follows.Fu×L1=(L1+L2)×FsFu={(L1+L2)/L1}×mεω2Obtained hence is the relationship of Fu∝ω2.
On the other hand, in case of rotating the rotor at a high speed, an elastic shaft is used as the rotation drive shaft. The elastic shaft described herein is a shaft which is elastically deformed, e.g., bent within a range of operational rotation speed. Flexural rigidity of the rotation drive shaft is lowered in the elastic shaft, so that the flexural natural frequency is within a low-speed range during rotation. That is, the elastic shaft is arranged so that the reaction force due to imbalance is reduced in the high-speed side. More specifically, FIG. 13 shows a rotating situation in which a rotor R2 is coaxially connected to an elastic shaft S2, and the elastic shaft S2 is coaxially connected to an output shaft of a motor M. At this time, the output shaft of the motor M is supported on a motor housing or the like (not shown) through a bearing B. Where the mass of the rotor R2 is m, deviation of an actual barycenter position from a geometric center is ε which is induced from imbalance of the rotor, the rotational angular speed is ω, flexure of the elastic shaft S2 is ρ, and flexural rigidity of the elastic shaft S2 is k, centrifugal force Fd induced by the imbalance is expressed as follows.Fd=m(ε+ρ)ω2=ρkFrom this expression, ρ is obtained as follows.ρ=mεω2/(k−mω2)Provided that the flexural natural frequency is represented by √(k/m)=ωn, k=mωn2 is given and the following is obtained.ρ=ε×(ω/ωn)2/{1−(ω/ωn)2}When ω/ωn=1 exists, i.e., when the rotation speed is equal to the flexural natural frequency (at the resonance point), the flexure becomes infinite. After the rotation speed exceeds the natural frequency, the flexure comes asymptotically close to ε. Therefore, after the rotation speed exceeds the natural frequency and the rotation speed further increases, the bearing reaction force Fu′ then takes the following relationship.Fu′∝εk
Differences in bearing reaction force between the high-rigidity shaft and the elastic shaft are shown in a graph in FIG. 14. In this graph, the ordinate axis represents the bearing load F and the abscissa axis represents the angular speed ratio. Since the high-rigidity shaft has high rigidity, it provides good usability. However, the bearing reaction force due to imbalance increases sharply as the rotation speed increases. On the other side, the elastic shaft has a resonance point at a low speed (the area of 1.0). However, stable rotation can be achieved at a high speed after the rotation speed exceeds the resonance point. Note that the flexure at the resonance point can be reduced to be low by providing an external damping mechanism.
In view of the foregoing, if an elastic shaft is adopted when both of a high-speed rotor and a bulky low-speed rotor are to be used in one centrifuge, stable rotation is obtained at a high speed. However, the rigidity of the shaft is so low that the rotation shaft easily bends if the bulky rotor is attached to the rotation shaft. In some cases, the rotation shaft may be broken. In addition, the rotor having a large volume causes great imbalance at samples treated by users, as described previously. Therefore, if the large swing rotor is rotated on an elastic shaft, the primary natural frequency (primary resonance point) of flexure of the rotation shaft becomes so large that the rotating rotor may contact fixed components or the rotation shaft may be bent and broken. To summarize the above, when an elastic shaft is adopted, the high-speed angle rotor provides good usability but the low-speed swing rotor provides adverse usability or cannot be mounted.
On the other side, when a high-rigidity shaft is adopted, the low-speed swing rotor provides good usability but bearing reaction force caused by imbalance of samples increases in proportion to square of the rotation speed. Therefore, no problem appears within a low-speed rotation range but the load to the bearing increases within a high-speed rotation range. Consequently, problems appear in that the lifetime of the bearing is shortened and the birling noise is large. It is hence inevitable to restrict the allowable amount of imbalance of samples to be small, and the user must take labor for adjusting amounts of samples in test tubes in uniform fashion. This is a factor of adverse usability. To summarize the above, when a high-rigidity shaft is adopted, the large low-speed rotor provides good usability while the high-speed rotor provides adverse usability.
The present invention has been made in view of the above drawbacks, and has an object to provide a centrifuge capable of selectively installing various rotors therein without sacrificing operability of the selected rotor.